Though it never said in the book, Chadley absolutely loves and adores strawberries, it's his favorite food, too. Because of the berry, the author thought the other characters should have a passion for other berries, too. Example, Sans, Papyrus and W.D. Gaster. Sans would adore blueberries, right know, the author just realized that Underswap Sans makes sense know. Papyrus would adore cherries, but adores spaghetti more. Then the author thought about Gaster for the longest time. Though nothing sparked an idea. Then it realized, dark, pitch-blackness...... black berries.